During normal processes of vascular growth (e.g., the menstrual cycle, placentation, changes in adiposity, wound repair, inflammation), the creation of new blood vessels is regulated and eventually ceases. Significantly, the deregulation of vascular growth is a critical element of pathology. For example, tumor growth, diabetic retinopathies, arthritis, and psoriasis involve excessive proliferation of blood vessels that contributes directly to the pathological state. In contrast, impairment of vascular growth, characteristic of aged individuals, compromises the healing of wounds and the revascularization of tissues rendered ischemic by trauma or disease. Therefore, an understanding of the mechanisms that direct the assembly new blood vessels, and the processes that start and stop vascular growth, are central to the development of strategies to control vascularization in disease.
During the growth of new blood vessels (angiogenesis), sprouts arise from endothelial cells that line the lumens of capillaries and postcapillary venules—the smallest branches of the vascular system. Angiogenesis is a complex, multi-step process. Although published studies of angiogenesis number in the many thousands, the cellular mechanisms that mediate and regulate angiogenic growth and morphogenesis are poorly understood.
The details of angiogenic sprouting are difficult to observe in “real-time” in vivo because of the opacity of most tissues. Tissue sections are difficult to reconstruct in 3D and do not communicate the dynamic nature of vascular growth. Moreover, the region near the tips of angiogenic sprouts—a critical area of control of vascular invasion and morphogenesis—is rarely found in tissue sections. In order to overcome the limitations of conventional histology, a variety of “models” of angiogenesis in vivo and in vitro have been developed.
Models of angiogenesis in vivo: To circumvent the opacity of living tissues, investigators have observed angiogenesis through “windows” in living animals that include the naturally transparent tails of amphibian larvae (Clark and Clark 1939), or specialized viewing chambers either implanted into rabbit ears (Clark and Clark 1939), mouse skin (Algire, Chalkley et al. 1945) and hamster cheek pouches (Greenblatt and Shubi 1968) or developed from rabbit corneal pockets (Gimbrone, Cotran et al. 1974) or chick chorioallantoic membranes (Ausprunk, Knighton et al. 1974). From these early, largely descriptive studies came validation of the central paradigm of tumor-induced vascular chemotaxis and the corresponding discovery of diffusible, tumor-derived molecules that promote vascular growth. Newer assays of angiogenesis in vivo measure vascular ingrowth into polymeric sponges or plugs of gelled basement membrane proteins implanted subcutaneously into rodents (Passaniti, Taylor et al. 1992; Andrade, Machado et al. 1997; Akhtar, Dickerson et al. 2002; Koike, Vernon et al. 2003). For all of their elegance, approaches in vivo are made difficult by: (1) intra-species variation in angiogenic response from animal to animal; (2) the lack of translation of results from one species to another; (3) high costs of animal purchase and maintenance; (4) public disapproval of the use of animals for research purposes; and (5) complexities encountered in animal surgeries and in the visualization and evaluation of results.Two-dimensional (2D) models of angiogenesis in vitro: In an effort to understand the molecular mechanics of angiogenesis, endothelial cells isolated from large vessels were cultured in flat dishes until they formed confluent, pavement-like monolayers that simulated the endothelial linings of blood vessels (Jaffe, Nachman et al. 1973; Gimbrone 1976). Although useful as models of proliferative responses to endothelial injury in large blood vessels (Gimbrone, Cotran et al. 1974; Fishman, Ryan et al. 1975; Madri and Stenn 1982; Madri and Pratt 1986; Jozaki, Marucha et al. 1990; Rosen, Meromsky et al. 1990), monolayer cultures of endothelial cells on rigid substrata do not typically organize into capillary-like tubes in simulation of angiogenesis. In 1980, however, following successful long-term culture of capillary endothelial cells (Folkman, Haudenschild et al. 1979), it was reported that 20-40 day cultures of bovine or human capillary endothelial cells developed a 2D cellular network on top of the confluent cellular monolayer, a process termed “angiogenesis in vitro” (Folkman and Haudenschild 1980). The endothelial cells of the network appeared as “tubes” with “lumens” filled with a fibrillar/amorphous material that was interpreted to be an endogenously-synthesized network of “mandrels” on which the cells organized. Later studies reported similar 2D network formation by endothelial cells from large vessels (Maciag, Kadish et al. 1982; Madri 1982; Feder, Marasa et al. 1983) and by endothelial cells seeded on top of malleable, hydrated gels of basement membrane proteins (e.g. Matrigel® gel)(Kubota, Kleinman et al. 1988).
Although 2D models of vascular development remain in use today (the Matrigel®-based assay (Kubota, Kleinman et al. 1988) is available commercially), such models lack the following 5 defining characteristics of true angiogenesis:    1. Invasion—Endothelial cells in 2D models form networks on top of extracellular matrix and show little propensity to burrow into the extracellular matrix (Vernon, Angello et al. 1992; Vernon, Lara et al. 1995).    2. Directionality—In 2D models, the networks of endothelial cells form in vitro more or less simultaneously throughout a field of pre-positioned cells, whereas angiogenesis in vivo involves the vectorial invasion of extracellular matrix by filamentous sprouts that arborize by multiple levels of branching.    3. Correct polarity—Although the 2D models make unicellular tubes that markedly resemble capillaries (Maciag, Kadish et al. 1982; Feder, Marasa et al. 1983; Sage and Vernon 1994), their polarity is “inside-out”, that is, they deposit basement membrane material on their luminal surfaces and have their thrombogenic surfaces facing outward to the surrounding culture media (Maciag, Kadish et al. 1982; Feder, Marasa et al. 1983)—opposite to the situation in vivo.    4. Lumen formation—Evidence that 2D models generate endothelial cell (EC) tubes with patent lumens is weak. Typically, the endothelial cell tubes have “luminal” spaces that are filled with extracellular matrix (either exogenous or synthesized by the cells) (Maciag, Kadish et al. 1982; Madri 1982; Feder, Marasa et al. 1983; Sage and Vernon 1994; Vernon, Lara et al. 1995). Where present, patent lumens usually appear as slit-like or narrow cylindrical spaces bounded by thick walls of endothelial cell cytoplasm—quite different from the inflated, thin-walled endothelial cell tubes that typify capillaries in vivo.    5. Cell specificity—The cellular networks in 2D models are generated by mechanical processes that may be accomplished by non-EC cell types (Vernon, Angello et al. 1992; Vernon, Lara et al. 1995). Indeed, mathematical modeling has shown that any adherent cell type capable of applying tensile forces to malleable, 2D extracellular matrix (either synthesized endogenously or supplied (e.g., Matrigel® gel)) can generate networks under optimal conditions (Manoussaki, Lubkin et al. 1996).Three-dimensional (3D) models of angiogenesis in vitro: The recognition that angiogenesis in vivo occurs within a 3D extracellular matrix has led to a variety of models in which sprouting is induced within 3D gels of extracellular matrix in vitro. In an early 3D model, endothelial cells dispersed within collagen gels (Montesano, Orci et al. 1983) formed networks of cords and tubes (Elsdale and Bard 1972). Although the endothelial cell tubes exhibited correct polarity, the characteristics of invasion and directionality were lacking (the endothelial cells were pre-embedded and evenly dispersed in the extracellular matrix). Nonetheless, this approach has proven useful in studies of lumen formation (Davis and Camarillo 1996) and of responses of endothelial cells to growth factors (Madri, Pratt et al. 1988; Merwin, Anderson et al. 1990; Kuzuya and Kinsella 1994; Marx, Perlmutter et al. 1994; Davis and Camarillo 1996).
In an alternative approach, 1 mm sections (rings) of rat aorta embedded in a 3D plasma clot generated branching, anastomosing tubes (Nicosia, Tchao et al. 1982). Sprouts from the aortic rings exhibited angiogenesis-like invasion and directionality in addition to polarity. Explant models utilizing aortic rings from rats or microvascular segments from mice have been used to study the influence of tumors, growth factors, various extracellular matrix supports, and conditions of aging on angiogenesis (Nicosia, Tchao et al. 1983; Mori, Sadahira et al. 1988; Nicosia and Ottinetti 1990; Nicosia, Bonanno et al. 1992; Villaschi and Nicosia 1993; Nicosia, Bonanno et al. 1994; Nicosia, Nicosia et al. 1994; Nicosia and Tuszynski 1994; Hoying, Boswell et al. 1996; Arthur, Vernon et al. 1998).
A variety of models exist that induce purified endothelial cells (as monolayers or aggregates) to sprout invasively into underlying or surrounding 3D extracellular matrix gels (Montesano and Orci 1985; Pepper, Montesano et al. 1991; Montesano, Pepper et al. 1993; Nehls and Drenckhahn 1995; Nehls and Herrmann 1996; Vernon and Sage 1999; Vernon and Gooden 2002). Each of these models has specific limitations that include difficulty in visualizing sprout formation, limited sprouting, a requirement for sectioning, or lack of effectiveness with certain types of endothelial cells.
Wolverine and Gulec have disclosed a 3D angiogenesis system (US 2002/0150879 A1) that involves embedding a fragment of tumor tissue into a matrix. The outgrowth of microvessels can be characterized to assay the angiogenic potential of the tissue. However, this approach does not provide luminal perfusion of the microvessels.
Neumann (the inventor here) et al. 2003, has disclosed the possibility of creating perfused microvessels in vitro that can be included in an artificial tissue. Neumann et al. 2003 teaches using 127 micrometer nylon fishing line as mandrels held by shrink tubing for making microvessels. The vessels were made from rat aortic smooth muscle cells embedded in agar. These microvessels were of an exploratory nature and not suitable for creating a human vessel graft.
Two-dimensional models of vascular growth in vitro do not establish the defining characteristics of angiogenesis listed previously, whereas existing 3D models reproduce some or most of the characteristics. Importantly, none of the 3D models currently available reconstruct a parent blood vessel that contains a pressurized, flowing, circulatory fluid. Consequently, none of the existing in vitro 3D models permit study of the contribution of luminal pressure and flow to vascular growth and morphogenesis.